Modulation of Bmp4 signalling in the epithelial–mesenchymal interactions that take place in early thymus and parathyroid development in avian embryos

Abstract

Epithelial-mesenchymal interactions are crucial for the development of the endoderm of the pharyngeal pouches into the epithelia of thymus and parathyroid glands. Here we investigated the dynamics of epithelial-mesenchymal interactions that take place at the earliest stages of thymic and parathyroid organogenesis using the quail-chick model together with a co-culture system capable of reproducing these early events in vitro. The presumptive territories of thymus and parathyroid epithelia were identified in three-dimensionally preserved pharyngeal endoderm of embryonic day 4.5 chick embryos on the basis of the expression of Foxn1 and Gcm2, respectively: the thymic rudiment is located in the dorsal domain of the third and fourth pouches, while the parathyroid rudiment occupies a more medial/anterior pouch domain. Using in vitro quail-chick tissue associations combined with in ovo transplantations, we show that the somatopleural but not the limb bud mesenchyme, can mimic the role of neural crest-derived pharyngeal mesenchyme to sustain development of these glands up to terminal differentiation. Furthermore, mesenchymal-derived Bmp4 appears to be essential to promote early stages of endoderm development during a short window of time, irrespective of the mesenchymal source. In vivo studies using the quail-chick system and implantation of growth factor soaked-beads further showed that expression of Bmp4 by the mesenchyme is necessary during a 24 h-period of time. After this period however, Bmp4 is no longer required and another signalling factor produced by the mesenchyme, Fgf10, influences later differentiation of the pouch endoderm. These results show that morphological development and cell differentiation of thymus and parathyroid epithelia require a succession of signals emanating from the associated mesenchyme, among which Bmp4 plays a pivotal role for triggering thymic epithelium specification.

A novel aspect of the structure of the avian thymic medulla

“…The "senescent" Hassall's bodies are invaded by vimentinpositive scavenger cells, such as heterophyl granulocytes and macrophages, which phagocytose, digest and eliminate the Hassall's bodies. The cysts develop from the epithelial progenitors but their development is stopped at an early stage (Dooley et al 2005) and therefore, the epithelial cells of the cysts maintain their polarized shape; this results in the absence of cross-talk between epithelial cells and T-cells (Klug et al 2002;Rossi et al 2007) and mesenchymal cells Itoi et al 2007;Neves et al 2012). We propose that the formation of both cysts and Hassall's bodies represents "developmental malformation" of the medullary epithelial cells, namely, the differentiation of epithelial progenitors is arrested in early and late stages, repectively.…”

“…Lack of a distinct mesenchymal cell population, which expresses platelet-derived growth factor receptor α (PDGFRα) results in diminished epithelial cell proliferation and subsequent hypoplastic thymus development ). Experiments of the last decade have confirmed not only the mesenchymal induction of TEC Itoi et al 2007;Neves et al 2012) but also that the epithelial-mesenchymal cross-talk is extended to epithelial-T-cell interaction (Klug et al 2002;Rossi et al 2007). A common bipotent progenitor of TEC exists not only in the early embryo but also persists in the postnatal and possibly in adult thymus (Bennett et al 2002;Bleul et al 2006;Farr et al 2002;Rossi et al 2007).…”

“…The three-dimensional thymic epithelial reticulum develops from the third and fourth branchial pouch endoderm (Farley et al 2013;Gill et al 2003;Gordon and Manley 2011;Le Douarin 1967;Le Douarin et al 1984;Neves et al 2012;Rodewald 2008;Venzke 1952) and differentiates into cortical and medullary epithelial cells. Le Douarin (1967) isolated the branchial pouch endoderm, before fusion with the ectoderm of the branchial groove and co-cultured it with the mesenchyme; this resulted in the formation of functional thymic tissue providing evidence that (1) branchial endoderm without ectoderm was capable of thymic organogenesis and (2) thymic epithelial cell (TEC) differentiation required mesenchymal cell cooperation.…”

Notch and Hedgehog in the thymus/parathyroid common primordium: Crosstalk in organ formation

“…In avian embryos, Gcm2 transcripts were first detected by RT-PCR in isolated quail (q) endoderm at embryonic day (E) 2.5 (25-30 somite-stage) (Neves et al, 2012). However, in situ expression of Gcm2 has only been observed in the anterior domain of the 3PP and 4PP at Hamburger and Hamilton Stage 18 (HH18) and HH22, respectively (Okabe and Graham, 2004).…”

“…Foxn1 transcripts were detected in isolated quail endoderm 24 hours after Gcm2 expression. At cE4.5, Foxn1 expression was observed in situ in the dorsal tip of the 3/4PP and transcription endures until birth (Neves et al, 2012). The gene is mutated in the nude mouse strain, which displays abnormal hair growth and failure of thymus development, leading to immunodeficiency (Nehls et al, 1996;Blackburn et al, 1996;Bleul et al, 2006).…”